With the development of society, financial service has played a very important role in people's daily life; hence the image acquisition of paper currencies is an indispensable important technique in the financial service field.
Currently, a common manner for acquiring an image of a paper currency is to use an image collecting sensor to scan the paper currency to acquire an image of the paper currency while the paper currency is driven by a driving wheel.
However, in a conventional image collecting device, the sheet-type medium may float greatly when being conveyed at a high speed, and the floating may cause a large change in a gray scale value of a collected image, and the image with the inaccurate gray scale value is adverse to the analysis processing after the acquisition process.